Project 5: HIV-1 genome stability and editing mediated by host proteins Summary In HIV-1 infected cells, many host proteins interact with viral RNA. These interactions can have positive or negative effects on viral replication. Some of these RNA protein interactions lead to editing of the genome, while others regulate RNA fate. CRNA investigators have discovered and characterized such several RNA:host protein interactions that form the basis for Project 5. This project will comprise a mix of structural and biological approaches to determine the precise nature and role of these important host protein-viral RNA interactions in HIV-1 replication. In Aim 1 Bieniasz and Smith will determine the X-crystal structure of a novel dinucleotide- sensing RNA-destabilizing antiviral protein bound to its target. This protein appears to impose a major selective pressure that drives the biased nucleotide composition of the HIV-1 genome, and likely the genomes of many other viruses. Its discovery has far reaching implications for the understanding of the selective forces driving nucleotide composition of viruses and their hosts and the detection of non-self RNA. For Aim 2, APOBEC3 proteins were shown by CRNA investigators to be incorporated into virions through interactions with viral RNA that appear to mimic the sequence preference of HIV-1 NC. Therefore, Bieniasz, Smith and collaborators will determine crystal structures of pigtail macaque and human APOBEC3H bound to an RNA target or a DNA substrate. Finally, Cullen has demonstrated that adenosine methylation at specific sites in the viral genome facilitates HIV-1 replication, and in Aim 3 of this project Cullen, Rouskin and Telesnitsky will determine the effect of m6A modification and m6A binding proteins on HIV-1 RNA structure and function, and their role in HIV- 1 replication.